The present invention relates to systems for eliminating volatile organic compounds (VOCs) used in various manufacturing processes. More particularly, the present invention relates to an energy-saving heat exchanger for saving fuel in such a system.
Industrial operations generate enormous quantities of hazardous air pollutants into the atmosphere around the world each year. These hazardous air pollutants include volatile organic compounds (VOCs) that are subject to emission control by various legislative bodies. VOCs have caused considerable environmental and health concerns in recent years due to their environment-polluting effects. For example, VOCs are precursors of ground-level ozone, which contributes to smog formation. Consequently, various manufacturing industries such as dry cleaners, bakeries, restaurants and microbreweries are being increasingly regulated with regard to effective and environment-preserving disposal of VOCs.
In the semiconductor production industry, various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include the deposition of layers of different materials including metallization layers, passivation layers and insulation layers on the wafer substrate, as well as photoresist stripping and sidewall passivation polymer layer removal. In modern memory devices, for example, multiple layers of metal conductors are required for providing a multi-layer metal interconnection structure in defining a circuit on the wafer. Chemical vapor deposition (CVD) processes are widely used to form layers of materials on a semiconductor wafer, while etching processes are used to etch a circuit pattern in a conductive layer after deposition of a masking layer on the conductive layer. Both CVD processes and etching processes generate VOCs which must be abated and vented from the semiconductor fab in an environmentally satisfactory manner.
One example of a conventional VOC abatement system utilized in the semiconductor fabrication industry for the treatment and elimination of VOCs is generally indicated by reference numeral 10 in FIG. 1. The system 10 includes a fresh air blower 12 which blows fresh air into a combustion chamber 14. A primary ignition fuel such as diesel oil is ignited in the combustion chamber 14 to heat the fresh air therein. The heated air is directed through an air intake line 18 to a primary heat exchanger 16, where process. gases including VOCs distributed from a VOC concentrator 28 through a gas service line 38 and the primary heat exchanger 16 are heated by thermal exchange with the heated air flowing through the primary heat exchanger 16. The heated process gases are distributed from the primary heat exchanger 16 through a gas entry line 40 and into the combustion chamber 14, where the process gases are used as a secondary ignition fuel in the combustion chamber 14 to heat the air from the fresh air blower 12.
The heated air flows from the primary heat exchanger 16 through an air transfer line 20 to a secondary heat exchanger 22, where heat is transferred from the heated air to process gases flowing from a process gas entry line 26 and a bypass line 34, respectively. The heated process gas flows from the secondary heat exchanger 22 and into the VOC concentrator 28 through a VOC entry line 36. Some of the heated process gas in the VOC concentrator 28 flows to the primary heat exchanger 16 and ultimately, to the combustion chamber 14 through the gas service line 38 and gas entry line 40, respectively. The rest of the heated process gas flows from the VOC concentrator 28 through a process gas outlet line 30 and finally, into a vent stack 44 through a vent stack entry line 42. The heated air from the secondary heat exchanger 22 flows into the vent stack entry line 42 through an air outlet line 24 and mixes with the process gas from the VOC concentrator 28 in the vent stack 44. The air and process gases are then vented from the vent stack 44.
In the conventional VOC abatement system 10, fresh air introduced into the combustion chamber 14 from the fresh air blower 12 has a temperature of typically about 25xc2x0 C., or room temperature. It has been found that heating the fresh air to a temperature of about 120xc2x0 C. or greater before introducing the air into the combustion chamber 14 substantially reduces the quantity of primary fuel required for heating the air in the combustion chamber 14.
Accordingly, an object of the present invention is to provide a device for saving fuel in a VOC abatement system.
Another object of the present invention is to provide a device for preheating fresh air before entry of the air into a combustion chamber in order to reduce the quantity of fuel required to heat the air.
Still another object of the present invention is to provide an energy-saving heat exchanger which is capable of efficiently heating air in order to reduce consumption of fuel used to heat the air in a VOC abatement system.
A still further object of the present invention is to provide an energy-saving heat exchanger which is capable of one of two modes of operation in the preheating of air introduced into a combustion chamber of a VOC abatement system.
Yet another object of the present invention is to provide an energy-saving heat exchanger which utilizes heated exhaust air to heat fresh air before entry of the fresh air into a combustion chamber of a VOC abatement system.
A still further object of the present invention is to provide an energy-saving heat exchanger which in a first mode of operation utilizes heated exhaust air to preheat fresh air before introducing the fresh air into a combustion chamber and which heat exchanger in a second mode of operation mixes the heated air with the fresh air and introduces the air mixture into a combustion chamber in the event that the fresh air has a minimum oxygen content.
In accordance with these and other objects and advantages, the present invention comprises an energy-saving heat exchanger for a VOC abatement system. The energy-saving heat exchanger includes a fresh air chamber for receiving fresh air from a blower; a heated air chamber for receiving heated air from a secondary heat exchanger of the VOC abatement system; a mixed air chamber provided at the outlet ends of the fresh air chamber and heated air chamber; and a heat exchange chamber leading from the heated air chamber and disposed in thermally-conductive contact with the fresh air chamber. An oxygen detector measures the oxygen composition of air flowing from the mixed air chamber and operates a damper which controls flow of air from the heated air chamber to the mixing chamber.
Normally, fresh air from the blower flows through the fresh air chamber and the mixed air chamber, respectively, and ultimately, into the combustion chamber of the VOC abatement system. When the oxygen composition in the fresh air flowing from the mixed air chamber is lower than a predetermined value, typically about 18%, the oxygen detector operates the actuator to completely close the damper and prevent flow of the oxygen-deficient heated air from the heated air chamber and into the mixed air chamber. Consequently, the heated air is averted from the heated air chamber and through the heat exchange chamber, wherein the fresh air flowing through the fresh air chamber is heated by the heated air flowing through the heat exchange chamber. Conversely, when the oxygen composition in the fresh air flowing from the mixed air chamber exceeds a predetermined oxygen concentration, such as about 18% oxygen concentration, the oxygen detector operates the actuator to open the damper and facilitate flow of heated air from the heated air chamber and into the mixed air chamber. This facilitates maximum heat exchange between the heated air and the fresh air as the mixed air flows from the mixed air chamber and is directed to the combustion chamber of the VOC abatement system. Consequently, the mixed air in the combustion chamber requires less fuel for heating than would otherwise be required for fresh air entering the combustion chamber at room temperature.